The dissolution of plastics comprising polymers has long been known and practiced. For instance, a plastic is dissolved in a solvent to prepare a solution, which is applied to the surface of a solid objective to be coated with this plastic or is used in adhering two or more solid objects. The dust of a plastic contaminating the surface of a solid object can be dissolved by a solvent for cleaning. A plastic, mixed with other non-plastic materials as an impurity, can be removed by dissolving it.
The economically viable and sustainable recycling of discarded plastics has been an extremely challenging problem. This problem can be attributed to the fact that discarded plastics are almost always mixed or commingled with other plastics comprising various polymers. Nevertheless, economic motivations, environmental constraints, regulatory requirements and/or societal desires urgently demand that the used or discarded plastics be recovered or recycled to the maximum extent possible. Moreover, it is almost always preferable or necessary that any discarded mixture of plastics be separated into individual plastics prior to recycling for reuse. In the two most common methods, a discarded or waste mixture of plastics to be reused is crushed to form coarse particles or pellets, which are subsequently pyrolyzed or melted and intensively shear-mixed in their entirety. The former yields a fuel gas and/or oil, and the latter produces homogeneous solid blocks. Unfortunately, both methods yield relatively low-value added products of low quality.
Different mechanical methods have been proposed or deployed to separate mixtures of discarded plastics into individual plastics, each comprising one variety of polymer. Some of major methods include, manual sorting, sieving, filtration, flotation, air cycloning, and hydrocycloning, and combinations thereof. These methods are effected by different physical properties and/or morphological characteristics of plastics in the mixtures, such as density, size, shape, and combinations thereof. Often, however, it is extremely difficult to achieve clean separation between various plastics due to the closeness of their physical properties and/or morphological characteristics as well as due to the contamination by non-plastic materials, e.g., metals, papers and soils. Furthermore, it is nearly impossible to separate a mixture of plastics sharply, i.e., with high resolution, if any one of plastics itself is a mixture. Examples of such plastics are a multilayered plastic, composite plastic comprising interpenetrating polymers, and composite plastic formed by ultra high-pressure fusion.
Methods of selective dissolution have been proposed or deployed to circumvent the above-mentioned difficulties encountered in separating physically mixed or commingled solid plastics comprising various polymers by purely mechanical means. These methods take advantage of the fact that the solubilities of plastics are dissimilar in different organic solvents, and they vary differently with temperature. Some selective dissolution methods for separating physically mixed or commingled plastics into individual plastics, each comprising a single polymer, have been developed on the basis of the above-mentioned solubility characteristics of plastics. Methods have also been developed to dissolve an individual plastic or polymer with one or more solvents. Such methods deploy a wide variety of organic solvents.
U.S. Pat. No. 3,701,741 discloses a process of recovering substantially pure polyethylene terephthalate from contaminated scrap polyethylene terephthalate via solvent dissolution under high pressure and at temperatures ranging from ambient temperature to about 250° C. The solvents deployed are aliphatic alcohols, including methyl alcohol, ethyl alcohol, normal propanol, isopropanol, and mixtures thereof.
U.S. Pat. No. 4,003,881 discloses a process of recovering polyester polymer for reuse from the discarded dyed polyester fibers by solvent dissolution and dye-stripping. The solvents deployed are para-chloroanisole; dichloromethane; nitrobenzene; 1,1,1 trichloroethane; acetophenone; trichloroacetic acid; propylene carbonate; dimethyl sulfoxide; 1,1,2,2 tetrachloroethane; 2,6-dimethyl phenol; quinoline; 1,1,1,3,3,3 hexafluoro-isopropanol; ethylene carbonate; naphthalene; propylene carbonate; meta-cresol; chloroform; phenol; carbon tetrachloride; tetrahydronaphthalene; ortho-phenylphenol; para-phenylphenol; trifluoroacetic acid; ortho-chlorophenol; trichlorophenol; diphenyl; diphenyl ether; methyl naphthalene; benzophenone; diphenyl methane; dimethyl formamide; benzyl alcohol; para-dichlorobenzene; acenaphthene; phenanthrene; ethylene glycol; 1,2,2 trifluoroethane; and/or paradichlorobenzene.
U.S. Pat. No. 4,137,393 discloses a process representing a slight modification of U.S. Pat. No. 4,003,881. The solvents claimed include: para-chloroanisole; nitrobenzene; acetophenone; dimethyl sulfoxide; 2,6-dimethyl phenol; quinoline; naphthalene; meta-cresol; phenol; tetrahydronaphthalene; ortho-phenylphenol; para-phenylphenol; trifluoroacetic acid; ortho-chlorophenol; trichlorophenol; diphenyl; diphenyl ether; methyl naphthalene; benzophenone; diphenyl methane; dimethyl formamide; para-dichlorobenzene; diphenyl methane; acenaphthene; phenanthrene; a solvent characterized by at least one carbocyclic ring; naphthalene; diphenyl; diphenyl ether; methyl naphthalene; benzophenone; diphenyl methane; phenanthrene; acenaphthene; para-dichlorobenzene; and naphthalene.
U.S. Pat. No. 4,031,039 discloses a method for separating plastics comprising different polymers in a discarded mixture by resorting to the aforementioned selective dissolution. The solvents deployed in the examples or specifically claimed in the patent include: o-xylene; p-xylene; m-xylene; tetrahydrofuran (THF); cyclohexanone; dioxane; and methylethylketone; as well as each of said organic solvents containing water, an alcohol CnH2n+1OH (wherein n=1, 2, 3 or 4), inorganic acid; organic acid, inorganic alkaline compound, or organic basic compounds, either singly or in a combination of two or more.
U.S. Pat. No. 5,198,471 discloses a method of separating plastics of different polymers in a physically commingled mixture based on the aforementioned selective dissolution. Specifically, a first plastic, or polymer, is dissolved at a lower temperature and the remaining plastics, or polymers, are dissolved sequentially at higher temperatures. The method is illustrated by examples deploying solvents including tetrahydrofuran (THF), xylene, toluene, and ethylene glycol.
U.S. Pat. No. 5,278,282 is a continuation-in-part of U.S. Pat. No. 5,198,471, mentioned above. This patent discloses a method of heating, separating and dissolving polymers from a commingled solid mixture. The solvents used include: tetrahydrofuran; toluene; xylene; N-methylpyrrolidinone; n-butanol; cyclohexanol; N, N-dimethyl acetamide; 1-methyl 2-pyrrolidinone; amyl acetate; 2(2-butoxyethoxy) ethanol; chlorobenzene; cyclohexane; cumene; decahydronaphthalene; diethyl maleate; tetrahydronapthalene; cyclohexanone; 1,2-dichlorobenzene; 2-undecanone; and mixtures thereof.
French Patent 2776663 discloses a process of recovering substantially pure polyvinyl chloride (PVC) from plastic items via solvent dissolution. The solvents deployed are methyl-ethyl-ketone (MEK), methyl-isobutyl-ketone (MIBK), and tetrahydrofuran (THF).
An exhaustive survey of available technical literature, including journal articles, conference proceedings articles, presentations, books, and monographs, has revealed wide ranging varieties of organic solvents, some of which are included in the above-mentioned patents, for dissolution of plastics. These solvents include: 1,1,2,2-tetrachloroethane; 1,2-dichlorobenzene; 1,2-dichloroethane; 1,2-dichloropropane; 1,3-dichlorobenzene; 1,3-dioxane; 1,4-dichlorobutane; 1,4-dioxane; 2-butanone; 2-methylcyclohexanone; 2-nonanone; 2-picoline; 3-methylcyclohexanone; 3-nonanone; 3-pentanone; 4-heptanone; 5-nonanone; a mixture of bromobenzene, isoamyl acetate decalin and isoamyl acetate; acetone; acetylacetone; all aliphatic alcohols; anisole; anthracene; bean oil (vegetable oils in general); benzaldehyde; benzothiopene; benzyl alcohol; benzyl chloride; bromobenzene; butyl formate; chlorobenzene; chlorofluoro-carbons (CFCs); chloronaphthalene alcohol (MeOH or EtOH)+ether (dioxane, THF or dimethoxy-ethane); chorinated solvents (e.g., 1,2-dichloroethane); cyclohexanone; cyclopentane; cyclopentanone; cyclopentyl chloride; decalin; diethyl phthalate; dihydronaphthalene; diisopropyl ketone; dimethyl naphthalene; dimethyl phthalate; dimethyl-acetamide/diglyme (88% wt DMAC); dipropylene glycol; ethyl acetate; ethyl acetoacetate; ethylbenzene; ethylene dichloride (EDC); ethylene glycol (EG); fluorene; fluorobenzene; highly paraffinic “white” oil (e.g., recycled lubricating oil base starch); isobutyl methyl ketone; isopropethylalcohol (IPA) (propa-2-ol or 2-propanol); isopropyl acetate; isopropyl methyl ketone; isopropylbenzene; ketones in general; m-chlorotoluene; MEK with one of six non-solvents (water, methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol); methyl acetate (MA); methyl acetoacetate; methyl Cellosolver® acetate; methylene chloride; methyl isobutyl ketone (MIBK); N,N-diethylaniline; N,N-dimethylaniline; N,N-dimethylformamide; naphthalene acetonitrite alcohol (methanol, ethanol, 2-propanol, 1-butanol or hexanol); N-cyclohoexyl-2-pyrrolidinone (CHP); N-ethyl-2-pyrrodinone (NEP); n-hexadecane; nitrobenzene; N-methyl-2-pyrrolidine xylene-cyclohexane mixture; N-methyl-2-pyrrolidone (NMP); o-chlorotoluene; o-xylene; pentanol; perylene; phenetole; propyl acetate; pyrene; styrene (monomer in general); tetraethylene glycol; tetrahydrofuran; tetrahydrofurfuryl alcohol; tetrahydropyran; tetralin; toluene; and toluene-methanol mixture.
Thus, a substantial number of organic solvents has been proposed for the dissolution of single-polymer or multi-polymer plastics alone or in a physically commingled mixture. Apparently, however, few of these solvents have been extensively deployed commercially. This could be attributed to one or more of the following characteristics, or properties, of such solvents: (1) highly volatile, thus entailing the dissolution to be conducted under an appreciable high pressure even at a relatively low or moderately high temperature, (2) unacceptably high toxicities and/or low flash points, (3) difficult to manufacture in mass, (4) non-recyclable, and/or (5) non-renewable.
Unconsolidated pieces of a material dissolve far faster in a liquid state than in a solid state at any given temperature, e.g., a material after and before melting, respectively, as long as they do not become consolidated as molten mass or masses upon melting, thereby reducing the surface area available for mass transfer or dissolution.
According to prior methods and apparatuses, unconsolidated plastics in various forms, e.g., powder, particles, pellets and irregularly shaped, crushed scraps, under high shearing and/or compressing forces, are melted with the aid of heating to yield a molten mass. Upon exiting from an apparatus, this molten plastic mass almost immediately solidifies, forming a consolidated, plastic object. It is inconceivable that either the molten mass or subsequently solidified object of plastic can be readily reverted back to the unconsolidated form in the dissolving solvent. Moreover, the apparatus for melting can be costly. Some examples of patents disclosing the above methods and apparatuses are: U.S. Pat. No. 4,218,146, which discloses an extruder-like apparatus to melt a thermoplastic material; U.S. Pat. No. 4,388,262, which is the modification of the preceding patent; and U.S. Pat. No. 5,240,656, which discloses an apparatus and method to melt a plastic by passing it through the controlled heating zone by gravity flow.
The solvents most commonly used in laboratories or deployed commercially for the dissolution or selective dissolution of plastics comprising polymers or other polymeric materials because of their availability include: acetone; hexane; methanol; ethanol; butanol(s), propanol(s), hexanol(s); ethylene dichloride (EDC); tetrahydrofuran (THF); toluene; xylene(s); cumene; cyclohexane; and ethylene glycol. The boiling points of these solvents range from 56° C. for acetone to 197° C. for ethylene glycols. These boiling points are within the range of about 100° C.±60° C. except that of ethylene glycol, and thus, they are unsuitable for melting plastics under ambient or near ambient pressure. The melting points of the majority of discarded plastics that are recycled, including polyvinyl chloride (PVC), polystyrene (PS), low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), and polyethylene terephthalate (PET), are within about the same temperature range as that of the boiling points of these solvents or somewhat higher.